1. Field of Invention
The present invention relates to a power amplifier for use in audio equipment and the like, and more particularly to an auto bias circuit for a power amplifier using a power MOSFET, which can compensate for a drain current in the power amplifier using the power MOSFET.
2. Description of the Prior Art
Referring to FIG. 1, there is shown the configuration of a fixed bias circuit for a power amplifier using a power MOSFET.
The power MOSFET has extremely low input impedance for high frequency, due to its large input capacity. Therefore, an impedance conversion transformer T11 having a winding ratio of 2:1 (i.e., impedance ratio of 4:1) is used to provide low resistance of a signal source so as to drive the power MOSFET Q11.
The power MOSFET Q11 has a source to which a source resistor R16 is connected for enhancing stability of a DC bias current. The power MOSFET has a drain and a gate between which a negative feedback is provided.
Further, the negative feedback implements a broadband amplifier and a reduction of output impedance. Here, the provision of the negative feedback should be determined in consideration of both a bandwidth and a power gain. Furthermore, a slight broadbanded effect may be obtained by dividing feedback resistors and then connecting a capacitor C13 to a ground.
The power MOSFET, which is primarily used as a switching element, has very large forward transconductance, and thus, a single power MOSFET can be used as a practical power amplifier for audio equipment.
Such switching element can be used for the fixed bias in the same manner as the prior art. However, since characteristics of a semiconductor device greatly vary with change in temperature, temperature compensation is needed. Moreover, since the power MOSFET element has a very large value of transconductance gts, there is a large difference between characteristics of individual elements. If the power MOSFET element is used as the switching element, it does not cause any problems. However, if the power MOSFET element is used in the power amplifier for the audio equipment, individual adjustments should be made for each element. Therefore, there is a disadvantage in that the use of the fixed bias may remarkably degrade productivity of the power amplifier upon mass production thereof and increase production costs. Furthermore, there is a problem in that inconvenience is caused by the need for manual adjustment of a variable resistor VR11.
The present invention is conceived to solve the above problems. An object of the present invention is to provide an auto bias circuit for a power amplifier using a power MOSFET, which allows the power amplifier to always perform an ideal operation by automatically compensating for a drain current and to be mass-produced at high quality and low cost.
According to one aspect of the present invention for achieving the above object, there is provided an auto bias circuit for a power amplifier using a power MOSFET, comprising an emitter-grounded NPN type transistor having a collector connected to a DC source and to a gate of the power MOSFET, and a base connected to a source of the power MOSFET through a resistor for protecting the NPN type transistor and removing an AC component in cooperation with a capacitor; a smoothing capacitor of which one end is grounded and which is connected to the collector of the NPN type transistor; and an additional resistor of which one end is grounded and which is connected to the source of the power MOSFET and to the resistor for protecting the NPN type transistor and removing the AC component, thereby detecting a drain current.
According to another aspect of the present invention, there is provided an auto bias circuit for a power amplifier using a power MOSFET, comprising a PNP type transistor having a collector connected to a gate of the power MOSFET through a diode side of a photocoupler, a base connected to a drain of the power MOSFET through a resistor for protecting the PNP type transistor and removing an AC component in cooperation with a capacitor, and an emitter connected to a voltage source; a smoothing capacitor of which one end is grounded and which is connected to a collector of the photocoupler between a DC source and the gate of the power MOSFET; and an additional resistor of which one end is connected to the voltage source and which is connected to the drain of the power MOSFET, thereby detecting a drain current.